In operation, a disk drive controller generates a command to move the head to a certain location, and the command is translated into drive voltage signals applied to the winding of a VCM mounted at one extremity of the movable arm. These drive signals may be linear or switched mode signals. Switched mode drivers may implement different techniques such as, for example, pulse width modulation (PWM), phase shift modulation (PSM) and others, Generally, switched mode drivers may have the advantage of reducing power dissipation in the output transistors that comprise a common output bridge stage to which the motor coil is connected, and thus allow smaller devices and packaging, as compared to linear mode drivers.
The drive signals that are applied to the VCM functioning in a PSM mode are illustrated in FIG. 1. The PSM output power signals OUT1 and OUT2 are driven to a high logic level Vcc (where Vcc is power supply voltage), when PSM input signals are at high digital level, and to a low logic level equal to the ground voltage, when PSM input signals are at low digital level. In a PSM driver, a null output current may be obtained by driving both OUT1 and OUT2 with a 50% duty cycle. The ripple on the output current, Iload, is also shown. The functioning of a PSM driver and its advantages are disclosed in U.S. Pat. No. 5,917,720 to Galbiati.
The main advantage of PSM driver technique may be its outstanding linearity of control under any condition. In fact, as will be appreciated by the person of ordinary skill in the art, the PSM technique may not suffer, like the PWM technique, of the “Ton minimum” problem. The “Ton minimum” problem may not allow good control of the output voltage below a certain value of the duty cycle. Moreover, when PSM drives an inductive load like a VCM, the load current ripple decreases as the load current decreases toward zero, and this may be a benefit when there is need to implement very precise control under relatively low output current levels, as is typically the case in disk drives.
FIG. 2 illustrates a photographic reproduction of a typical disk drive mechanical arrangement. FIG. 2 also illustrates the VCM moving the R/W head carrying arm and the flexible flat cable connection (Flex Cable) carrying the power signal lines (PSM OUT1 and PSM OUT2!) as well as the low level signal lines (R/W head signals), running parallel in the flat cable. However, certain characteristics of the PSM driver for applications, such as, the one illustrated in FIG. 2 and alike, may be found to degrade the signal-to-noise ratio (S/N). These characteristics may manifest especially during precise controlling of the position of the R/W head over the rotating disc, a phase during which the resulting output current that is applied to the PSM driven coil of the VCM is relatively low level.
In fact, when controlling the VCM at relatively low current levels, both PSM drive signals may approach the 50% duty cycle, and, therefore, the PSM commutations may become so close to each other that they may practically be considered concurrent. In these conditions of operation, switching energies may sum and reach a level that may be sufficient to inject switching noise from the VCM power lines into adjacent low level signals lines of the flexible flat cable of electrical connection of the moving arm.
FIGS. 3 and 4 illustrate simulation waveforms when the commutations of the two PSM drive signals are not concurrent and concurrent, respectively. As may be observed in the latter case, the noise due to the two commutations concentrates in a narrowing time zone, and the resulting energy level may increase to a point sufficient to inject noise into adjacent low level signals lines of the flexible cable, corrupting data being transferred through the R/W channel.